In the last few years, numerous development have become known which deal with replacing the filter materials made from mono- or multilayer papers or fabrics which have been known for a long time in the state of the art. Thus dust filter bags are described in EP 1 258 277 B1 and in EP 0 960 645 B1, in which a composite made of nonwoven materials is used as filter material. The composite thereby comprises at least two layers, these layers having different filtration properties.
Furthermore, a filter medium made of a nonwoven material is described in EP 1 197 252 A1 in which electrostatically charged split fibres are connected by ultrasonic welding to form a compacted web made of nonwoven material. In order also to endow the layer material with corresponding stability, it is thereby provided that at least 5% of the surface is provided with weld points relative to the entire surface of the filter material and that at least two weld points per cm2 are present. It is consequently achieved that the individual fibres are connected directly to each other by ultrasonic welding. It is mentioned as an advantage of a filter medium of this type that the production costs relative to a needling method are significantly less and that consequently uniformity is produced so that an improvement is achieved relative to those filter media which are connected for example by needling.
It is provided furthermore also in the above-mentioned European application that this thus stabilised filter medium can be connected also to further nonwoven layers.
It is disadvantageous with the filter medium of EP 1 197 252 A1 above all that the dust storage capacity for house dust is simply inadequate.
Furthermore, an electrostatic filter medium which comprises 2 to 20 meltblown layers (ultrafine nonwoven fibre layers) is known from U.S. Pat. No. 5,900,305. The meltblown layers are thereby connected to each other by welds. It is unfavourable in the case of the filter medium according to U.S. Pat. No. 5,900,305 that, for use in dust filter bags in which the medium to be filtered out is present in the most varied of particle sizes, it does not have a sufficient dust storage capacity.